The present invention relates to communication systems using non-geosynchronous satellites and, more particularly, to a system which determines and registers the locations of mobile terminals.
There has been proposed, as one of next-generation mobile satellite communication systems, some mobile or personal communication systems using multi-spot beam satellites in non-geosynchronous orbits. In these communication systems, a plurality of satellites are put into orbits at an altitude lower than those for geosynchronous orbiting satellites of conventional satellite communications systems, that is, into a low earth orbit (in the altitude range of 500 to 2,000 kilometers) or medium earth orbit (in the altitude range of 10,000 to 35,000 km) and to link the satellites with mobile terminals through spot beams.
Communications systems using non-geosynchronous orbiting satellites, as described above, have advantages such as low propagation loss, short propagation delay, a simple satellite configuration, and a smaller number of spot beams required for each satellite than that for a multi-spot beam satellite in the geosynchronous orbit.
FIG. 8 is a conceptual diagram of the communication between a mobile terminal and a land earth station in the mobile satellite communication system. In FIG. 8, reference number 1 denotes a mobile terminal, 2 a land earth station, 30 a multi-spot beam satellite capable of communicating with the mobile terminal 1 and the land earth station 2, and 30-1 through 30-7 individual spot beam coverage areas formed or defined by dividing an satellite coverage area.
Upon occurrence of an incoming call to a mobile terminal in such mobile satellite communication systems, it is necessary to transmit paging message to connect a call to the called terminal. To minimize the number of channels required for paging, it is desirable that areas for transmitting paging message be set as small as possible.
The Inmarsat standard-A system adopts a method that a caller designates the ocean region, by dialing, where the called mobile terminal is expected to be located at the time of originating a call. When areas for paging are wide, the number of areas to be selected by the caller is small but the large number of channels for transmitting paging messages is required, whereas when the areas for paging are narrow, the number of channels required for paging is small but the burden on the caller increases. Hence, it is particularly impractical to apply this method to communication systems in which the number of subscribers is large. It is therefore desirable to implement a method or scheme wherein, as in the terrestrial cellular systems, the communication system accurately keeps tracks of locations of all pageable mobile terminals. For example, by managing the information in databases of a terrestrial network or the like and allows the caller to call a desired mobile terminal just by dialing a number assigned to the terminal equipment or its user. For a system using multi-spot beam satellites in the geosynchronous orbit, it is possible to determine and register the locations of mobile terminals in the same manner as in the terrestrial cellular systems. Namely, a coverage area of each spot beam irradiated from satellites can be utilized as the location information of mobile terminals.
In the system using non-geosynchronous orbiting satellites, however, since the satellite 30 rotates around the earth, the satellite coverage areas 30-1 through 30-7, over which mobile terminals can communicate with the satellite 30, and the spot beam coverage areas move on the surface of the earth, accordingly. Hence if spot beam coverage areas are used as the location information of mobile terminals, the location information will be updated very frequently, and so, many channels will be required to register and update the location information, which is not practical.